The present invention relates to a cord comprising a core structure and one layer. Said core structure consists of 1 to 4 core filaments having a diameter d1, d1 being greater than 0.15 mm and smaller than 0.40 mm, said layer consists of n+m layer filaments having a diameter d2 and being twisted in the same direction and at the same pitch as the core filaments of said core, d2 being greater than or equal to d1xe2x88x920.05 mm and being smaller than or equal to d1xe2x88x920.015 mm, n being equal to the number of core filaments and m being equal to six if n is greater than one and being equal to five if n is equal to one. The present invention also relates to a rubber article, and more particularly a tire and a breaker layer of a tire comprising a cord as described hereabove.
It is well known that cords, especially steel cords, may be used for reinforcement of rubber articles. Such steel cords conveniently, but not exclusively, comprise steel filaments having a carbon content of more than 0.60% carbon, e.g. more than 0.68%, or more than 0.81%. A typical steel composition is: a minimum carbon content above 0.65%, a manganese content between 0.40% and 0.70%, a silicon content between 0.15% and 0.30%. The sulphur and phosphor contents are preferably kept below 0.03%. Other, more expensive elements such as chromium may also be alloyed. The elongation at rupture of steel filaments used for rubber reinforcement preferably lies above 1%, most preferably above 2.5%. These steel filaments conveniently have a tensile strength above 2200 N/mm2, preferably above 2700 N/mm2.
Compact cords occupy a special place among the cords. All the filaments of these cords, with exception of a possible wrapping filament, are twisted in the same direction and at the same pitch. These compact cords have several highly appreciated advantages. They are manufactured in one single step as is explained in the UK Patent Application GB-A-2 028 393. So cost of manufacturing is low. Another advantage is that the filaments composing the compact cords have line contacts with each other. This reduces fretting. And as the name itself suggests, compact cords have a high compactness, i.e. their cross-sections have compact configurations having a maximum of filled surface per occupied cross-sectional surface. However, compact cords have the drawback of a risk of core migration, especially when these cords are used in the breaker of a tire.
One well known solution to avoid core migration is to use different diameters for the filaments: the core structure of the compact cords consists of 1 to 4 filaments having a diameter d1 which is 0.015 to 0.05 mm greater than the diameter d2 of the other filaments forming a layer surrounding the core structure. If the difference between the two diameters is smaller than 0.015 mm, then the risk for core migration becomes greater. If the difference between the two diameters is greater than 0.05 mm then the degree of compactness decreases.
This solution has proved to be a good solution to prevent core migration but only in case of correct filament location, i.e. if the thicker filaments (diameter d1) form the core structure, and the thinner filaments (diameter d2) form the layer. If there is a bad filament location the risk of core migration still remains. This bad filament location may be caused by an erroneous exchange of spools by the labourer in the unwinding unit of the twisting machine or by an alternation in the filament position during the twisting process.
The small difference in diameter (0.015-0.05 mm) between the core and the layer filaments is not sufficient to be detected with the naked eye. Hence, checking whether the filament location of a twisted compact cord is correct, is a manual and time-consuming job. Furthermore, the checked sample is no longer usable: A sample of 1 m cord is cut from a bobbin. The labourer disentangles the different filaments of this sample manually and looks for the core filaments. The core filaments are different from the layer filaments in that their deformation degree is lower due to a lower distance from the imaginary axis of the cord. Once the core filaments are identified their diameters are measured manually with a micrometer. The result is subsequently noted down. The time needed for one such measurement varies from 2 to 2.5 minutes. If the application of the cord is critical, e.g. if the cord is to be used as a breaker reinforcement of a tire, such measurement is needed per winding unit (e.g. per 16 kg bobbin).
This costly and time-consuming check detects possible erroneous exchanges of spools in the unwinding unit but does not guarantee a 100% correct filament location over the whole length of the cord.
It is an object of the present invention to avoid the drawbacks of the prior art. It is a further object of the present invention to provide a compact cord, the time needed for checking the filament location of said compact cord being minimized.
According to a first aspect of the present invention there is provided a cord comprising a core structure and one layer, aid core structure consisting of 1 to 4 core filaments having a diameter d1, d1 being greater than 0.15 mm and smaller than 0.40 mm, said layer consisting of n+m layer filaments having a diameter d2 and being twisted in the same direction and at the same pitch as the core filaments of said core, d2 being greater than or equal to d1xe2x88x920.05 mm and being smaller than or equal to d1xe2x88x920.015 mm, n being equal to the number of core filaments and m being equal to six if n is greater than one and being equal to five if n is equal to one characterized in that the core filaments have a surface which is visually different from the surface of layer filaments. The cord may or may not be surrounded by a single wrapping filament. Visually different means that it is detectable with the naked, unaided eye of a person with ordinary visual capacities. A substantially different colour is a visual difference.
The visual difference of the different categories of filaments may be realized in several ways. In the case of steel cord used for the reinforcement of rubber articles all the filaments are conveniently coated with a brass alloy in order to provide the necessary rubber adhesion. One way to make the visual difference is to avoid coating the core filaments with a brass alloy or any other metal or alloy, while still coating the layer filaments with a brass alloy or with copper. The difference in colour between the naked core filaments and the brass-coated layer filaments is more than sufficient to be detected with the naked eye of the labourer. Another way is to coat the core filaments with zinc, aluminium, cobalt, tin, magnesium or nickel, and to coat the layer filaments with a brass alloy. Among these zinc is appreciated as a coating of the core filaments since it has acceptable drawing properties. The core filaments may also be coated with copper, but the visual difference with the layer filaments which are brass-coated is not that great.
A third way may be to surround a thin coloured nylon fiber around the core filaments and not around the layer filaments. In this case the core filaments may be coated with brass too.
It is clear that with a cord according to the first aspect of the present invention the time-consuming check of the filament location becomes superfluous and that a visual check lasting only some instants is sufficient. It is also clear that a bad filament location over a limited distance due to an alternation in the filament position during the twisting process may easily be discovered too. The check may also be automatized.
According to a second aspect of the present invention there is provided a rubber article comprising a cord according to the first aspect of the present invention. The cord according to the first aspect of the present invention is preferably used in one or more breaker layers of a tire.